The present invention relates to the detection of call progress signaling tones on telephone lines, primarily in user telephone equipment but also in other telephone equipment such as central office or PBX equipment.
Telephone systems use recognizable call progress tones to indicate to the listener the status of a component of the system. Examples include: dial tone, busy tone, reorder tone, and ringback tone. The dial tone is a steady tone and the other three employ an on-off cadence. These tones are made to sound sufficiently different to the listener that the listener can distinguish one from another.
From time to time, the invention of additional features for telephone systems has required the adoption of a novel tone to indicate another status. For example, U.S. Pat. No. 4,221,933 issued to R. Cornell on Sep. 9, 1980 discloses the use of a "distinctive dial tone", such as "an interrupted or stuttered dial tone" to indicate to a customer that a voice message is waiting which can be accessed by dialing a special number. Similarly, U.S. Pat. No. 4,918,719 issued to Daudelin in April 1990, discloses the use of a "stuttered dial tone" as a signal meaning the user wants to dial a third number for a conference call.
The signaling tones described above are all intended for communicating with a human user of the telephone system. As originally conceived, they all require that the user be capable of hearing and distinguishing the various tones. However, in some telephone system equipment it is desired that call progress tones be detected by a device connected to the telephone line which device then causes other actions. For example, various items of telephone system equipment, such as central office switches and PBX switches, can handle additional sophisticated functions for customers if they can recognize and distinguish the various tones. Various methods of distinguishing distinctive tones are known in the art. For example, U.S. Pat. No. 4,935,958 issued to S. Morganstein on Jun. 19, 1990, discloses a device which examines the frequency and cadence of the tone, can learn the characteristics of a tone, and can identify a tone as the same as a previously learned tone. Various actions can then be electronically initiated when a distinctive tone is recognized. However, the currently known methods of identifying and distinguishing the various tones generally require equipment which is prohibitively expensive for wide distribution among individual customers.
In particular, electrical devices for identifying an on-off cadence in the call progress tone, such as for a busy tone, a ringback tone, a reorder tone, or a stuttered dial tone, are complicated and expensive. The most common method is to use precision components to look at a single cycle of the on-off cadence and accurately measure the duration of the on period and the duration of the off period. These periods are then compared to expected values, typically by using a microprocessor, to identify the tone. Similar methods are used in electrical applications other than telephone systems.
To meet the widely perceived need for a visual indication that a message is waiting for a customer, in lieu of or in addition to the stuttered dial tone, one company has developed a feature called "Message Waiting Indication--Visual". When a message is waiting for a telephone system customer, this feature illuminates an indicator light on the customer's telephone equipment. This system requires special equipment at the central office which can send a special electronic signal to the customer's telephone equipment and special customer equipment which can receive and interpret the signal and turn on the indicator light. This design has substantial limitations including: (1) cost, (2) incompatibility with the audible message waiting indication so one cannot determine whether a message is waiting by lifting the hand set of an extension telephone, (3) incompatibility with certain other telephone system features, and (4) loss of the visual message waiting indication signal if the customer's telephone is "off-hook" for 2 -1/2 hours or more when the signal is sent.
Special customer premises telephone equipment, such as the message waiting indication device described above, typically requires electrical power to perform the additional features. If the power requirements are low and are needed only when the telephone is "off-hook" the power can be obtained from the loop current supplied by the central office. However, various features, such as the indicator light described above, require power when the telephone is "on-hook". In these situations, power is obtained from batteries which must be periodically replaced or from a plug connected to a standard AC wall outlet.